Creating archives

arca create <OUT> <INPUTS>... [-l LEVEL] [-c CODEC] [-j THREADS] [-p PASSWORD] [--hide-names]

Levels

arca create copy.zip my-files/ -l store    # no compression, fastestarca create copy.zip my-files/ -l fastarca create copy.zip my-files/ -l normal   # the defaultarca create copy.zip my-files/ -l best     # smallest output

Codecs

auto picks Deflate for .zip, because a zip exists so that anything can open it. Ask for Zstandard explicitly when you control the receiving side:

arca create copy.zip my-files/ -c zstd

Threads

Arca uses every core by default. Pin the count with -j, for example on shared CI runners:

arca create copy.zip my-files/ -j 8arca create copy.zip my-files/ -j 0    # every core (the default)

Scaling measured 1.79× on two threads, 90% efficiency, against a design requirement (R3) of at least 1.6×. See benchmarks for the command.

7z

arca create copy.7z my-files/ -l normalarca create stored.7z my-files/ -c storearca create secret.7z my-files/ -c lzma2 -p "a password" --hide-names

The levels store, fast, normal and best select Copy or LZMA2 levels 1, 6 and 9. -c store overrides the level. Only auto, store and lzma2 are accepted for 7z; explicit Deflate/Zstandard codec selection applies to ZIP, not 7z. TAR ignores those codec choices and .tar.gz always uses gzip; lzma2 is rejected outside 7z. Creation is sequential regardless of -j, with one independent block per file, not solid compression. Reading existing solid archives is supported.

7z includes empty files and directories and rejects symlink/reparse-point and special-file inputs rather than following them. Creation writes a temporary archive beside the output and replaces it only on success; failure preserves an existing output. The output's parent must exist. If all inputs are empty, 7z encryption requires --hide-names so the password can actually be checked.

RAR

arca create backup.rar documents/ notes.txt -l bestarca create stored.rar big.iso -c store

Creates a new single-volume, non-solid, unencrypted RAR5 archive that official UnRAR and 7-Zip open. The four levels select the RAR compressor strength; -c store equals -l store and any other explicit codec is refused. Creation is sequential (-j above 1 is refused), stores empty files and directories, and rejects symbolic links and special files instead of following them.

RAR output is never encrypted (-p and --hide-names are refused), never a .cbr, never solid or multivolume, and never replaces an existing file: the archive is staged beside the output, verified with Arca's reader and published only if the name is still free. Existing RAR/CBR archives stay read-only. Limits and the complete list of refusals are in the RAR guide.

TAR and gzip

arca create copy.tar my-files/arca create copy.tar.gz my-files/ -l best

The gzip layer honours the chosen level. TAR has nowhere to put encryption, so -p is refused for .tar and .tar.gz.

XZ

arca create copy.tar.xz my-files/ -l best   # .txz works tooarca create notes.txt.xz notes.txt          # one file, no container

An .xz is a compressed stream, not an archive: it holds the bytes of exactly one file and no name, date or folder. Arca refuses a folder or more than one input for it and suggests .tar.xz, which puts a TAR inside the XZ stream. Extracting notes.txt.xz writes notes.txt; a file without the suffix gets .out appended.

The levels map to xz presets: store is preset 0, fast 1, normal 6 and best 9. Even preset 0 compresses with LZMA2; XZ has no stored mode. Only auto and lzma2 are accepted as codecs, and -p and --hide-names are refused. The output carries a CRC64 check that xz and tar -J verify.

-j sets how many blocks are compressed at the same time. Each worker needs the memory of its preset, so Arca uses as many as fit in 2 GiB (one at best, up to 14 at normal) and prints the number actually used. With more than one worker the stream is split into independent blocks, which costs a little ratio. Reading is a single sequential stream. Creation writes a temporary file beside the output and renames it only on success.

What gets stored

• Directories are walked recursively in sorted order, so the same input always produces the same entry order.
• Paths are stored relative to the parent of each input: arca create a.zip ~/work/site stores entries as site/….
• Modification times are preserved.
• ZIP/TAR add regular files and skip symbolic links. 7z and RAR also preserve empty directories and reject symbolic links (see above).

Encrypting while you create

arca create secret.zip folder/ -p "a password"

See Encryption for the scheme, its guarantees and its limits.

Output

silesia.zip: 12 files, 202.1 MB -> 63.1 MB (68.8% smaller) in 0.282 s · 717 MB/s · 24 threads